At this point, the curious professional will ask the question "Why not make an unlicensed LPWAN system synchronous?". There are synchronisation elements in the LoRaWAN protocol - this is the operation of devices in class "B".. There are synchronous UNB systems in the field of security equipment - this is the "Basalt'' system produced by Altonica. There are active attempts to develop new LPWAN technologies based on synchronous channels, e.g. SNBWAN protocol.
Synchronous systems have an obvious advantage - the gateway controls the access parameters of each ED (endpoint device) to the network, which permits a significant increase in bandwidth and reduces the waiting time of message transmission on the return channel.
However, synchronous systems have a darker side of the coin, which is the necessity to synchronise all the EDs. In synchronous LPWANs, the leading role in controlling all operating tactics is given to the return channel of the system, which strictly specifies the operating mode of each endpoint device.
The side effects of synchronisation in an unlicensed LPWAN would be:
1.Poor tolerance of the channel to external influences. The synchronisation channel is constantly running, it is easily tapped, and any disturbance leads to catastrophic consequences in the entire system. An intruder only needs to corrupt a small piece in the synchronisation channel (it can be done without breaking any laws) and the network falls down.
2.The problem with synchronising gateways between themselves. In class "B" LoRaWAN, the gateways are very precisely timed to each other, they only transmit time beacons simultaneously on the same frequency and their signals overlap each other - so there will always be a dead zone where they overwhelm each other and the ED will not hear any. The frequency and time separation of the gateway signals makes the system very difficult to organise, especially in dynamics - we come to a cellular structure with all the associated costs.
3. Synchronisation of moving EDs. In synchronous systems, each ED has to be constantly monitored and controlled, difficulties arise with every new device in the gateway operation zone, this becomes critical for moving objects from gateway to gateway - the ED has to constantly register in the new gateway zone, and this is at least additional energy consumption and radio air load.
4.Overlapping the reception area of the gateways. The operating zones of neighbouring gateways usually overlap with each other and a message from one ED may be received by several gateways simultaneously - it is not clear how to effectively organise synchronisation so that neighbouring devices do not interfere with another gateway.
5.Security. The synchronisation channel is almost impossible to encrypt. This is a vulnerable element for network security attacks.
6. Radio interference. The constant operation of a gateway transmitter located at a high point spreads the signal over a large coverage area, and creates orders of magnitude more interference in the surrounding air than interference from end devices located at ground level or indoors.
The overhead of using synchronisation in LPWAN is too high and synchronous systems are used where it is justified. For example, class “B” in LoraWAN is now practically not used by anyone.
To increase the throughput of the uplink channel of an asynchronous LPWAN, however, it is enough to install another additional diversity gateway, to increase both the system's throughput and the reliability of its operation due to parallel reception
